sftp: check directory is empty before issuing rmdir

Some SFTP servers allow rmdir on full directories which is allowed
according to the RFC so make sure we don't accidentally delete data
here.

See: https://forum.rclone.org/t/rmdir-and-delete-empty-src-dirs-file-does-not-exist/7737

backend/sftp/sftp.go

@@ -594,6 +594,16 @@ func (f *Fs) Mkdir(dir string) error {
 
 // Rmdir removes the root directory of the Fs object
 func (f *Fs) Rmdir(dir string) error {
+	// Check to see if directory is empty as some servers will
+	// delete recursively with RemoveDirectory
+	entries, err := f.List(dir)
+	if err != nil {
+		return errors.Wrap(err, "Rmdir")
+	}
+	if len(entries) != 0 {
+		return fs.ErrorDirectoryNotEmpty
+	}
+	// Remove the directory
 	root := path.Join(f.root, dir)
 	c, err := f.getSftpConnection()
 	if err != nil {